Outer Standard Disc Research Articles

The SOUX AGN sample: optical/UV/X-ray SEDs and the nature of the disc

ABSTRACT We use the SOUX sample of ∼700 active galactic nucleus (AGN) to form average optical-ultraviolet (UV)-X-rays spectral energy distributions (SEDs) on a two-dimensional (2D) grid of MBH and L2500. We compare these with the predictions of a new AGN SED model, qsosed, which includes prescriptions for both hot and warm Comptonization regions as well as an outer standard disc. This predicts the overall SED fairly well for 7.5 < log(MBH/M⊙) < 9.0 over a wide range in L/LEdd, but at higher masses the outer disc spectra in the model are far too cool to match the data. We create optical-UV composites from the entire Sloan Digital Sky Survey sample and use these to show that the mismatch is due to there being no significant change in spectral shape of the optical-UV continuum across several decades of MBH at constant luminosity. We show for the first time that this cannot be matched by standard disc models with high black hole spin. These apparently fit, but are not self-consistent as they do not include the General Relativistic effects for the emission to reach the observer. At high spin, increased gravitational redshift compensates for almost all of the higher temperature emission from the smaller inner disc radii. The data do not match the predictions made by any current accretion flow model. Either the disc is completely covered by a warm Comptonization layer whose properties change systematically with L/LEdd, or the accretion flow structure is fundamentally different to that of the standard disc models.

Origins of the UV/X-ray relation in Arakelian 120

ABSTRACTWe explore the accretion geometry in Arakelian 120 using intensive UV and X-ray monitoring from Swift. The hard X-rays (1–10 keV) show large amplitude, fast (few-day) variability, so we expect reverberation from the disc to produce UV variability from the varying hard X-ray illumination. We model the spectral energy distribution (SED) including an outer standard disc (optical), an intermediate warm-Comptonization region (UV and soft X-ray), and a hot corona (hard X-rays). Unlike the lower Eddington fraction AGN (NGC 4151 and NGC 5548 at L/LEdd ∼ 0.02 and 0.03, respectively), the SED of Akn 120 (L ∼ 0.05LEdd) is dominated by the UV, restricting the impact of reverberating hard X-rays by energetics alone. Illumination from a hard X-ray corona with height ∼10 Rg produces minimal UV variability. Increasing the coronal scale height to ∼100 Rg improves the match to the observed amplitude of UV variability as the disc subtends a larger solid angle, but results in too much fast variability to match the UV data. The soft X-rays (connected to the UV in the warm-Comptonization model) are more variable than the hard, but again contain too much fast variability to match the observed smoother variability seen in the UV. Results on lower Eddington fraction AGN have emphasized the contribution from reverberation from larger scales (the broad-line region), but reverberation induces lags on similar time-scales to the smoothing, producing a larger delay than is compatible with the data. We conclude that the majority of the UV variability is therefore intrinsic, connected to mass-accretion rate fluctuations in the warm-Comptonization region.

QPOs in compact binaries from small-scale eruptions in an inner magnetized disc

ABSTRACT Dwarf novæ (DNe) and low-mass X-ray binaries (LMXBs) are compact binaries showing variability on time-scales from years to less than seconds. Here, we focus on explaining part of the rapid fluctuations in DNe, following the framework of recent studies on the monthly eruptions of DNe that use a hybrid disc composed of an outer standard disc and an inner magnetized disc. We show that the ionization instability, which is responsible for the monthly eruptions of DNe, is also able to operate in the inner magnetized disc. Given the low density and the fast accretion time-scale of the inner magnetized disc, the ionization instability generates small, rapid heating and cooling fronts propagating back and forth in the inner disc. This leads to quasi-periodic oscillations (QPOs) with a period of the order of 1000 s. A strong prediction of our model is that these QPOs can only develop in quiescence or at the beginning/end of an outburst. We propose that these rapid fluctuations might explain a subclass of already observed QPOs in DNe as well as a, still to observe, subclass of QPOs in LMXBs. We also extrapolate to the possibility that the radiation pressure instability might be related to type B QPOs in LMXBs.

NuSTAR/XMM–Newton monitoring of the Seyfert 1 galaxy HE 1143-1810

Aims. We test the two-corona accretion scenario for active galactic nuclei in the case of the “bare” Seyfert 1 galaxy HE 1143-1810. Methods. We perform a detailed study of the broad-band UV–X-ray spectral properties and of the short-term variability of HE 1143-1810. We present results of a joint XMM–Newton and NuSTAR monitoring of the source, consisting of 5 × 20 ks observations, each separated by 2 days, performed in December 2017. Results. The source is variable in flux among the different observations, and a correlation is observed between the UV and X-ray emission. Moderate spectral variability is observed in the soft band. The time-averaged X-ray spectrum exhibits a cut-off at ∼100 keV consistent with thermal Comptonization. We detect an iron Kα line consistent with being constant during the campaign and originating from a mildly ionized medium. The line is accompanied by a moderate, ionized reflection component. A soft excess is clearly present below 2 keV and is well described by thermal Comptonization in a “warm” corona with a temperature of ∼0.5 keV and a Thomson optical depth of ∼17 − 18. For the hot hard X-ray emitting corona, we obtain a temperature of ∼20 keV and an optical depth of ∼4 assuming a spherical geometry. A fit assuming a jet-emitting disc (JED) for the hot corona also provides a nice description of the broad-band spectrum. In this case, the data are consistent with an accretion rate varying between ∼0.7 and ∼0.9 in Eddington units and a transition between the outer standard disc and the inner JED at ∼20 gravitational radii. Conclusions. The broad-band high-energy data agree with an accretion flow model consisting of two phases: an outer standard accretion disc with a warm upper layer, responsible for the optical–UV emission and the soft X-ray excess, and an inner slim JED playing the role of a hard X-ray emitting hot corona.

Modelling the spectral energy distribution of super-Eddington quasars

ABSTRACT We develop a broad-band spectral model, agnslim, to describe super-Eddington black hole accretion disc spectra. This is based on the slim disc emissivity, where radial advection keeps the surface luminosity at the local Eddington limit, resulting in L(r) ∝ r−2 rather than the r−3 expected from the Novikov-Thorne (standard, sub-Eddington) disc emissivity. Wind losses should also be important but these are expected to produce a similar radiative emissivity. We assume that the flow is radially stratified, with an outer standard disc, an inner hot Comptonizing region and an intermediate warm Comptonizing region to produce the soft X-ray excess. This gives the model enough flexibility to fit the observed data, but with the additional requirement of energy conservation to give physical constraints. We use this to fit the broad-band spectrum of one of the most extreme Active Galactic Nuclei, the Narrow Line Seyfert 1 RX J0439.6−5311, which has a black hole mass of $(6\sim 9)\times 10^6\, \mathrm{M}_\odot$ as derived from the H β line width. This cannot be fit with the standard disc emissivity at this mass, as even zero spin models overproduce the observed luminosity. Instead, we show that the spectrum is well reproduced by the slim disc model, giving mass accretion rates around (5 ∼ 10) × Eddington limit. There is no constraint on black hole spin as the efficiency is reduced by advection. Such extreme accretion rates should be characteristic of the first Quasars, and we demonstrate this by fitting to the spectrum of a recently discovered super-Eddington Quasar, PSO J006 + 39, at z = 6.6.

Rapid black hole growth at the dawn of the Universe: a super-Eddington quasar atz = 6.6

We present the analysis of a new near-infrared (NIR) spectrum of a recently discovered $z=6.621$ quasar PSO J006+39 in an attempt to explore the early growth of supermassive black holes (SMBHs). This NIR (rest-frame ultraviolet, UV) spectrum shows blue continuum slope and rich metal emission lines in addition to Ly$\alpha$ line. We utilize the MgII line width and the rest frame luminosity $L_\text{3000\AA}$ to find the mass of SMBH ($M_\text{BH}$) to be $\sim 10^8 M_\odot$, making this one of the lowest mass quasars at high redshift. The power-law slope index ($\alpha_\lambda$) of the continuum emission is $-2.94\pm0.03$, significantly bluer than the slope of $\alpha_\lambda=-7/3$ predicted from standard thin disc models. We fit the spectral energy distribution (SED) using a model which can fit local SMBHs, which includes warm and hot Comptonisation powered by the accretion flow as well as an outer standard disc. The result shows that the very blue slope is probably produced by a small radial ($\sim230$ gravitational radius, $R_\text{g}$) extent of the standard accretion disc. All plausible SED models require that the source is super-Eddington ($L_\text{bol}/L_\text{Edd} \gtrsim 9$), so the apparently small disc may simply be the inner funnel of a puffed up flow, and clearly the SMBH in this quasar is in a rapid growth phase. We also utilize the rest-frame UV emission lines to probe the chemical abundance in the broad line region (BLR) of this quasar. We find that this quasar has super solar metallicity through photoionization model calculations.

OUP accepted manuscript

Recent monitoring campaigns designed to map the accretion regime in AGN show major discrepancies with models where the optical/ultraviolet (UV) is produced by X-ray-illuminated, optically thick disc material within a few hundred gravitational radii. However, these campaigns only monitored X-rays below $10$ keV, whereas the bolometric luminosity for most of these AGN peaks above $50$ keV. We use data from the recent multiwavelength campaign by \cite{E17} on NGC 4151 - the only AGN bright enough to be monitored at higher energies with \textit{Swift} BAT. We develop a spectral-timing model with a hot corona, warm Comptonisation, and outer standard disc. This fits the time-averaged spectrum well, but completely fails to match the UV variability predicted from the X-ray lightcurve. However, it reveals that NGC 4151 had a bolometric luminosity around $1.4\%$ of the Eddington luminosity during this campaign, close to the luminosity at which AGN show a `changing-state' transition, where the broad optical lines disappear. Stellar mass black holes show a state transition at a similarly low Eddington fraction, which is broadly interpreted as the inner disc being replaced by an optically thin flow. We find that the UV lightcurve can instead be matched by reprocessing of the X-ray flux on size scales of the broad line region (BLR; $1.5-20$ light-days) and rule out there being optically thick material inwards of this, as expected if the thin disc is replaced by the flow below the inner radius of the BLR. These results emphasise the need for even longer-timescale, multiwavelength monitoring campaigns on variable AGN.

A physical model of the broad-band continuum of AGN and its implications for the UV/X relation and optical variability

We develop a new spectral model for the broadband spectral energy distribution (SED) of Active Galactic Nuclei (AGN). This includes an outer standard disc, an inner warm Comptonising region to produce the soft X-ray excess and a hot corona. We tie these together energetically by assuming Novikov-Thorne emissivity, and use this to define a size scale for the hard X-ray corona as equal to the radius where the remaining accretion energy down to the black hole can power the observed X-ray emission. We test this on three AGN with well defined SEDs as well as on larger samples to show that the average hard X-ray luminosity is always approximately a few percent of the Eddington luminosity across a large range of Eddington ratio. As a consequence, the radial size scale required for gravity to power the X-ray corona has to decrease with increasing Eddington fraction. For the first time we hardwire this into the spectral models, and set the hard X-ray spectral index self consistently from the ratio of the hard X-ray luminosity to intercepted seed photon luminosity from the disc. This matches the observed correlation of steeper spectral index with increasing Eddington ratio, as well as reproducing the observed tight UV/X relation of quasars. We also include the reprocessed emission produced by the hot inner flow illuminating the warm Comptonisation and standard disc regions and show that this predicts a decreasing amount of optical variability with increasing Eddington ratio as observed, though additional processes may also be required to explain the observed optical variability.

Tracking the energetics of the non-thermal disc–corona–jet in the very high state GX 339 − 4

The dramatic hard-soft spectral transition in Black Hole Binaries is important as it is associated with the collapse of the jet and with the strongest low frequency QPOs. These transition spectra (intermediate and very high state: VHS) are complex, with soft but distinctly non-thermal Comptonisation which merges smoothly into the disc emission. Here we develop a physical model for the accretion flow which can accommodate all these features, with an outer standard disc, which can make a transition to an energetically coupled disc-corona region, and make a further transition to a hot inner flow which can be radiatively inefficient if required. The code explicitly uses fully relativistic emissivity (Novikov-Thorne), and all Comptonisation is calculated with a hybrid (thermal and non-thermal) electron distribution. We fit this to a VHS spectrum from GX 339-4. We show that the complex continuum curvature produced by a hybrid electron distribution is enough to remove the strong constraint on black hole spin derived from reflection using simpler Comptonisation models. More fundamentally, we show that the VHS cannot be fit with the same Novikov-Thorne emissivity which can fit the disc dominated spectrum but instead requires that the inner flow is somewhat radiatively inefficient. This is consistent with an accretion powered jet, but simultaneous radio data show that the jet has already collapsed at the time of our data. Instead, it could point to truncation of the inner flow at radii larger than the innermost stable circular orbit, as predicted by the Lense-Thirring QPO models.

Multiwavelength light-curve evolution of Swift J1357.2−0933 during its 2011 outburst

Swift J1357.2-0933 underwent an episodic accretion in 2011 and provided very regular temporal and spectral evolution, making it an ideal source for exploring the nature of very faint X-ray transients (VFXTs). In this work, we present a detailed analysis on both X-ray and near-ultraviolet (NUV) light curves. The fluxes at all wavelengths display a near-exponential decays in the early phase and transits to a faster-decay at late times. The e-folding decay time-scales monotonically decrease with photon energies, and the derived viscous time-scale is $\tau_{\rm \dot{M}} \sim 60$ days. The time-scale in the late faster-decay stage is about a few days. The high ratio of NUV luminosity to X-ray luminosity indicates that the irradiation is unimportant in this outburst, while the near-exponential decay profile and the long decay time-scales conflict with the disc thermal-viscous instability model. We thus suggest that the disc is thermally stable during the observations. Adopting the truncated disc model, we obtain a lower limit of peak accretion rate of $0.03 \dot{M}_{\rm Edd}$ and the X-ray radiative efficiency $\eta < 5\times10^{-4}$, which decreases as the luminosity declines. The low X-ray radiative efficiency is caused by the combined action of advection and outflows, and naturally explains that the X-ray reprocessing is overwhelmed by the viscous radiation of the outer standard disc in the NUV regime. We also propose a possibility that the outer standard disc recedes from the central black hole, resulting in the faster-decay at late times.

What Kinds of Accretion Disks Are There in the Nuclei of Radio Galaxies?

It seems to be a widely accepted opinion that the types of accretion disks (or flows) generally realized in the nuclei of radio galaxies and in further lower mass-accretion rate nuclei are inner, hot, optically thin, radiatively inefficient accretion flows (RIAFs) surrounded by outer, cool, optically thick, standard-type accretion disks. However, observational evidence for the existence of such outer cool disks in these nuclei is rather poor. Instead, recent observations sometime suggest the existence of inner cool disks of the non-standard type, which develop in a region very close to their central black holes. Taking NGC 4261 as a typical example of such light-eating nuclei, for which both flux data ranging from radio to X-ray and data for the counterjet occultation are available, we examine the plausibility of such a picture for the accretion states, as mentioned above, based on model predictions. It is shown that the explanation of the gap seen in the counterjet emission in terms of the free–free absorption by an outer standard disk is unrealistic and, moreover, the existence itself of such an outer standard disk seems to be very implausible. Instead, the model of RIAF in an ordered magnetic field (so-called resistive RIAF model) can well serve to explain the emission gap in terms of the synchrotron absorption, as well as to reproduce the observed features of the overall spectral energy distribution (SED). This model also predicts that the RIAF state starts directly from an interstellar hot gas phase at around the Bondi radius, and terminates at the inner edge, whose radius is about 100-times the Schwartzschild radii. Therefore, there is a good possibility for a cool disk to develop within this innermost region.

A possible disk mechanism for the 23-day QPO in Mkn 501

Optically, thin two-temperature accretion flows may be thermally and viscously stable, but acoustically unstable. Here, we propose that the O-mode instability of a cooling-dominated optically thin two-temperature inner disk may explain the 23-day quasi-periodic oscillation (QPO) period observed in the TeV and X-ray light curves of Mkn 501 during its 1997 high state. In our model the relativistic jet electrons Compton upscatter the disk soft X-ray photons to TeV energies, so that the instability-driven X-ray periodicity will lead to a corresponding quasi-periodicity in the TeV light curve and produce correlated variability. We analyse the dependence of the instability-driven quasi-periodicity on the mass ( M) of the central black hole, the accretion rate ( M ˙ ) and the viscous parameter ( α) of the inner disk. We show that in the case of Mkn 501 the first two parameters are constrained by various observational results, so that for the instability occurring within a two-temperature disk where α = 0.05–1.0, the quasi-period is expected to lie within the range of 8–100 days, as indeed the case. In particular, for the observed 23-day QPO period our model implies a viscosity coefficient α ⩽ 0.28, a sub-Eddington accretion rate M ˙ ≃ 0.02 M ˙ Edd and a transition radius to the outer standard disk of r 0 ∼ 60 r g , and predicts a period variation δ P/ P ∼ 0.23 due to the motion of the instability region.

New flaring of an ultraluminous X-ray source in NGC 1365

We have studied a highly variable ultraluminous X-ray source (ULX) in the Fornax galaxy NGC 1365, with a series of 12 Chandra and XMM-Newton observations between 2002 and 2006. In 2006 April, the source peaked at a luminosity ≈3 x 10 40 erg s -1 in the 0.3-10 keV band (similar to the maximum luminosity found by ASCA in 1995), and declined on an e-folding time-scale ≈3 d. The X-ray spectrum is always dominated by a broad power-law-like component. When the source is seen at X-ray luminosities ≈10 40 erg s -1 , an additional soft thermal component (which we interpret as emission from the accretion disc) contributes ≈1/4 of the X-ray flux; when the luminosity is higher, ≈3 x 10 40 erg s -1 , the thermal component is not detected and must contribute <10 per cent of the flux. At the beginning of the decline, ionized absorption is detected around ∼0.5-2 keV; it is a possible signature of a massive outflow. The power law is always hard, with a photon index r ≈1.7 (and even flatter at times), as is generally the case with bright ULXs. We speculate that this source and perhaps most other bright ULXs are in a high/hard state: as the accretion rate increases well above the Eddington limit, more and more power is extracted from the inner region of the inflow through non-radiative channels, and is used to power a Comptonizing corona, jet or wind. The observed thermal component comes from the standard outer disc; the transition radius between outer standard disc and Comptonizing inner region moves further out and to lower disc temperatures as the accretion rate increases. This produces the observed appearance of a large, cool disc. Based on X-ray luminosity and spectral arguments, we suggest that this accreting black hole has a likely mass ∼50-150M ⊙ (even without accounting for possible beaming).

A unified accretion–ejection paradigm for Black Hole X-ray binaries

We present a unified accretion–ejection picture that explains the different spectral state of Black Hole X-ray binaries (BHXrB) from radio to X/γ-rays. In this view, the central region of BHXrB has a multi-flow configuration which consists in (1) an outer standard accretion disc, (2) an inner magnetized accretion disc driving, (3) a self-collimated electron–proton MHD jet, surrounding and (4) a relativistic electron–positron beam when adequate conditions are met. This picture provides a simple and unified explanation to the various canonical spectral states of BH X-ray binaries, by varying the transition radius r J between the inner disc driving jets and the outer standard disc. In this framework, large r J correspond to Quiescent and Hard states while small r J correspond to Thermal Dominant ones. In between these two extremes, r J can reach values that switches on the pair cascade process giving birth to a relativistic electron–positron beam. This would correspond to the bright intermediate state.

Spectral Energy Distribution of Bimodal Accretion Flows

We estimate the spectral energy distribution of bimodal accretion flows. These consist of an ADAF-like inner part connected to an outer standard disk through a narrow transition region. Previous works showed, that the transition region undergoes quasi-periodic variability on the timescale of the local keplerian orbital period. We obtain the electron temperature distribution from global models and estimate the spectral energy distribution. Additionally to the well-known spectral components from the ADAF and SSD, respectively, we find a new hot, optically thick component from the transition region. Further, we present synthetic lightcurves, showing the evolution during the quasi-periodic oscillation event.

Trapped Low‐Frequency Oscillations in the Transition Region between Advection‐dominated Accretion Flows and Standard Disks

We consider narrow transition regions in accretion disks where an outer standard disk (Shakura-Sunyaev-type disk; SSD) changes to an inner advection-dominated accretion flow (ADAF). By using the WKB method, we show that if the transition occurs in a relatively narrow region, trapped low-frequency oscillations, or slowly growing localized motions, are possible in the transition region. Their characteristic frequencies are much lower than the Keplerian frequency in this region. This type of trapped oscillation may be a possible cause of the 1-15 Hz quasi-periodic oscillations observed in GRS 1915+105.